Impacts of Suspended Sediment on Light Attenuation and Primary Productivity near the Elwha Delta

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Abstract

The Elwha River on Washington State’s Olympic Peninsula was home to two dams for almost a century. Dam removal began in 2011, releasing sediment into the Elwha River, forming a pronounced river plume. It is important to evaluate the response of the river ecosystem to such a drastic change. We examined light attenuation associated with the plume, using photosynthetically-active radiation measurements along with suspended sediment concentrations. Results suggest that there is not enough light penetrating depths of 5 m for successful primary production of some species. Suspended sediment in the water column prohibits light penetration > 10% water depth. As a result, the habitat of Z. marina, a local sea-grass species requiring a minimum of 18.6% light penetration, is likely affected. Based on suspended sediment concentrations in the water and the available light, it is hypothesized that Z. marina needs to grow at shallower depths away from the river plume in order to receive sufficient light in the Elwha delta.